‘Vidal blanc’ (Vitis vinifera × Vitis rupestris) is the most important white wine grape cultivar planted in Kentucky as a result of its adaptability to climate and soil and increased local demand. However, there is lack of information and research on grapevine canopy and cropload management for commercial production of ‘Vidal blanc’ under the long, warm growing season of the lower midwestern United States.
Grapevine crop control is commonly accomplished by balanced pruning. In interspecific hybrid cultivars such as Traminette (Vitis spp.), a balanced pruning formula of retaining 40 buds for the first 454 g of dormant wood removed and 10 additional buds for each additional 454 g of dormant wood resulted in optimum yield and fruit composition with minimal decrease in midwinter primary bud cold hardiness (O’Daniel et al., 2012). However, pruning is a rough regulator of yield and does not adequately control crop level or cropload of interspecific hybrids (Howell et al., 1987; Kaps and Cahoon, 1989; Kurtural et al., 2006). For example, ‘Chambourcin’ (Vitis vinifera × Vitis rupestris) produces a large number of fruitful shoots from non-count positions (Ferree et al., 2003; Kurtural et al., 2006). The fruitfulness of non-count buds is the primary cause of overcropping and ineffectiveness of balanced pruning (Kurtural et al., 2006; Pool et al., 1978). To remedy this problem and regulate cropping levels and cropload in several hybrid cultivars such as Chambourcin, Vidal blanc, and Seyval blanc, balanced pruning combined with cluster or berry thinning have been used together (Howell et al., 1987; Kaps and Cahoon, 1989; Kurtural et al., 2006).
Cluster thinning is a technique used to obtain the needed crop control in medium- and large-clustered interspecific hybrids (Kaps and Cahoon, 1989; Kurtural et al., 2006). Thinning to two clusters per shoot reduced yield but increased the total soluble solids of the grape juice at harvest and invigorated the vine in the following years in a number of French-American hybrid and V. vinifera cultivars (Bravdo et al., 1984; Nonnecke, 1980; Reynolds et al., 1986). Kurtural et al. (2006) reported acceptable crop reduction in ‘Chambourcin’ grapevines with a combination of 20 + 10 balanced pruning formula and cluster thinning to 1.2 clusters per shoot where yield was reduced but fruit quality was improved in addition to maintaining pruning weight per vine. For ‘Vidal blanc’, cluster thinning to one cluster per shoot and a balanced pruning formula of 15 + 10 was advised based on labor cost savings and minimized crop loss resulting from winter damage in Michigan (Howell et al., 1987). These results coupled with improved fruit quality and periderm browning of potted ‘Chambourcin’ led to a new concept, “balanced cropping,” where both balanced pruning, cluster thinning, and shoot thinning are used together for crop adjustment of interspecific hybrids as well as V. vinifera cultivars that can also be achieved mechanically (Geller and Kurtural, 2013; Kurtural et al., 2006, 2013; Main and Morris, 2004; Miller et al., 1996; Terry and Kurtural, 2011).
Vine balance in vineyards is commonly measured with the Ravaz Index (RI). The RI is the ratio between yield and pruning weight (Ravaz, 1911). The RI estimates the leaf area required to ripen clusters on the vine. Cropload management ameliorates the ratio between reproductive and vegetative growth of the grapevine to sustain productivity while optimizing yield, fruit composition, and midwinter, primary bud cold hardiness. Dormancy in grapevines is induced by shorter daylengths and colder temperatures and is essential if the vine is to withstand colder temperatures (Dami et al., 2005). The ability of the vine to enter a dormant state and to withstand freezing may be directly linked to the ratio between reproductive sinks and vegetative sources (O’Daniel et al., 2012).
Growers in Kentucky have had to rely on viticultural information generated in Michigan and New York. However, macroclimatic conditions in these regions are characteristic of cool climate viticulture (Jackson and Schuster, 1987) with growing degree-days (GDD) of 1250 GDD (base 10 °C) and 1275 GDD, respectively (Howell, 2001). Central Kentucky is characterized by warm climate viticulture with more than 2200 GDD accumulation (Kurtural et al., 2008). It was hypothesized that cropping targets for ‘Vidal blanc’ were not similar in cool vs. warm climate conditions. The long, warm growing season in Kentucky coupled with the typical 500 to 635 mm precipitation received during the growing season make proper management more challenging than the cooler and shorter growing seasons in the northern United States where ‘Vidal blanc’ has traditionally been grown. The long season (greater than 200 frost-free days) promotes more shoot growth per week, and the amounts of rain and humidity are key factors for development of fungal diseases as well. Thus, guidelines are needed for the lower Midwest that take into account the longer, warmer, and wetter growing season and the cold winters with fluctuating January temperatures.
The goal of this study was to identify optimal combinations of balanced pruning and cluster thinning levels without adversely affecting yield components, fruit composition, and midwinter primary bud cold hardiness of ‘Vidal blanc’ for commercial production in central Kentucky and the lower midwestern United States. The specific objectives were to determine the effect of pruning and cluster thinning on canopy architecture, yield components, and berry composition of ‘Vidal blanc’ grapevines.
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